Such sensor devices and methods are used for example to monitor and/or to control liquid levels.
Sensor devices are known, for example, which are designed as pivotably mounted floats and comprise a movable solid body. If the liquid, as it rises, reaches the level of the float, the latter is pivoted so that the solid body is displaced and actuates a switch. Such floats have the disadvantage that they come into contact with the liquid and under some circumstances become so soiled that they no longer function reliably.
Pneumatically acting sensor devices are also known, cf. for example U.S. Pat. No. 5,953,954. In this case, a feed line having an outlet is provided, via which compressed air can be fed as a feed substance. The outlet of the feed line is located in the liquid in order to sense the level thereof. The higher the liquid reaches above the outlet, the higher the hydrostatic pressure at the outlet and thus the higher the counterpressure acting on the air in the feed line. By sensing the pressure in the feed line, it is ultimately possible to draw a conclusion about the level of the liquid. This measurement principle has the disadvantage that it is relatively slow and a specific, predefined level of the liquid cannot be precisely sensed. It is also disadvantageous that, when a switch, for example a membrane switch, which switches at a specific pressure and thus at a specific level is installed in the feed line, usually an overpressure valve has to be provided in order to protect the switch against an overpressure. This may occur, for example, if the outlet of the feed line is blocked and thus the feed substance in the form of the compressed air is unable to flow away.